1. Field of the Invention
The present invention relates to illumination optical systems.
2. Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate or part of a substrate. A lithographic apparatus can be used, for example, in the manufacture of flat panel displays, integrated circuits (ICs) and other devices involving fine structures. In a conventional apparatus, a patterning device, which can be referred to as a mask or a reticle, can be used to generate a circuit pattern corresponding to an individual layer of a flat panel display (or other device). This pattern can be transferred onto all or part of the substrate (e.g., a glass plate), by imaging onto a layer of radiation-sensitive material (e.g., resist) provided on the substrate.
Instead of a circuit pattern, the patterning device can be used to generate other patterns, for example a color filter pattern or a matrix of dots. Instead of a mask, the patterning device can comprise a patterning array that comprises an array of individually controllable elements. The pattern can be changed more quickly and for less cost in such a system compared to a mask-based system.
A flat panel display substrate is typically rectangular in shape. Lithographic apparatus designed to expose a substrate of this type can provide an exposure region that covers a full width of the rectangular substrate, or covers a portion of the width (for example half of the width). The substrate can be scanned underneath the exposure region, while the mask or reticle is synchronously scanned through a beam. In this way, the pattern is transferred to the substrate. If the exposure region covers the full width of the substrate then exposure can be completed with a single scan. If the exposure region covers, for example, half of the width of the substrate, then the substrate can be moved transversely after the first scan, and a further scan is typically performed to expose the remainder of the substrate.
Illumination efficiency is an important parameter in maskless lithography, for example with regards to illuminating a diaphragm placed before a patterning device or illuminating the patterning device itself. A surface of the diaphragm or patterning device may only be comprised of 10-15% transmission areas or active areas, respectively, which should be illuminated, while having 85-90% non-transmission or inactive areas that should not be illuminated. Illuminating non-transmission or inactive areas unnecessarily uses up illuminating resources and can cause spurious reflections and stray light, which can interfere with and affect the characteristics of a patterned beam.
What is needed is a system and method that more efficiently illuminates a diaphragm and/or a patterning device, for example substantially only within a boundary of transmission areas of a diaphragm and/or active areas of a patterning device.